1. Field of the Invention
The present invention concerns a method to generate magnetic resonance exposures of an examination subject, in particular diffusion-weighted magnetic resonance exposures. “Magnetic resonance exposures” in this context means image data representing the interior of the examination subject generated with a magnetic resonance apparatus controlled in accordance with such a method, as well as parameter maps that reflect a spatial or temporal distribution of specific parameter values within the examination subject and that can be generated from the image data, for example. Moreover, the present invention concerns a control device for a magnetic resonance system as well as a magnetic resonance system with which such a method can be implemented.
2. Description of the Prior Art
Diffusion-weighted magnetic resonance exposures are magnetic resonance exposures with which the diffusion movement of specific substances (in particular water molecules) can be measured (detected) in the tissue of the body and can be shown with spatial resolution. Diffusion imaging has become established in the clinical routine, particularly for stroke diagnosis, since the stroke-susceptible brain regions are already apparent markedly earlier in diffusion-weighted images than in classical magnetic resonance exposures. Diffusion tensor imaging, in which the directional dependency of the diffusion is also detected, is a variant of diffusion-weighted magnetic resonance tomography. Diffusion-weighted magnetic resonance exposures herein encompass both magnetic resonance exposures generated within the scope of diffusion-weighted magnetic resonance tomography and magnetic resonance exposures generated within the scope of diffusion tensor imaging.
Diffusion-coded raw data must initially be acquired for the generation of diffusion-weighted magnetic resonance exposures. This takes place with special measurement (data acquisition) sequences that are designated as diffusion gradient measurement sequences in the following. In these measurement sequences, it is characteristic that, after a typical flipping of the relevant nuclear spins in one plane perpendicular to the basic magnetic field of the magnetic resonance scanner, a gradient magnetic field that varies the field strength of the external magnetic field in a predetermined direction is switched (activated) for a predetermined pulse length. The precessing nuclei thereby go out of phase, which is noticeable in the measurement signal.
Presently, diffusion-weighted exposures are typically made by a technique known as a “single shot” method. Within the pulse sequence, an excitation for the entire image (i.e. the complete spatial coding of an image) takes place after a single excitation pulse. One advantage of this method is that the phase effects used within the scope of the diffusion-weighted magnetic resonance tomography do not generate any additional movement artifacts.
An alternative is the use of measurement sequences in which multiple supplementary partial segments of k-space are acquired in succession in a “multi-shot” method, with the partial segments subsequently being combined. Methods known as “readout-segmented echo planar imaging” methods (rs-EPI methods) are among such measurement sequences. These methods have the advantage that information can be drawn from multiple exposures, and specific artifacts thus can be reduced. In addition, the image quality can be improved. In diffusion-weighted acquisition methods, however, it is precisely movements of specific substances that should be detected by utilizing the phase effects. If a data acquisition takes place with a multi-shot method and movements of the patient or of the organs occurs between the individual pulse sequences, this can lead to severe movement artifacts. Therefore, in such diffusion-weighted multi-shot methods, what is known as a navigator correction is implemented in which, each time raw data are acquired in a partial segment of k-space within the scope of a first echo, raw data are acquired from a middle k-space region in a subsequent second echo (known as the “navigator echo”), and thus a complete image with relatively low resolution is created. This resolution of the navigator image is good enough that the phase variations are perceptible in this image, and thus the images acquired in the navigator echo can be used in order to implement a correction of the individual shots based on one another.
In diffusion imaging, multiple images with different diffusion directions and weightings (i.e. with different diffusion coding gradient pulses) are normally acquired and combined with one another. The strength of the diffusion weighting is defined by what is known as the “b-value”. The different diffusion images, or the images or parameter maps combined from these diffusion images, can then be used for the desired diagnostic purposes. In order to be able to correctly estimate the influence of the diffusion movement, an additional reference exposure is normally necessary in which no diffusion coding gradient pulse is activated, i.e. an image with b=0. The pulse measurement sequence to acquire the reference data is designed in the same manner as the diffusion gradient measurement sequence, with the exception of the omission of the diffusion coding gradient pulses.
In addition to the diffusion-weighted images, T2-weighted exposures are still additionally generated in the typical clinical application since these show an important contrast for specific pathology information, in particular given tumors and strokes. T2*-weighted exposures are likewise frequently additionally produced since these have a higher sensitivity with regard to hemorrhages. Therefore, such T2*-weighted exposures are advantageous particularly in the case of a stroke in which the diffusion-weighted exposures are also of particular value.
It has previously been disadvantageous in the case of multi-shot diffusion-coded acquisitions that neither the pulse sequences for the diffusion-weighted images nor the pulse sequences for the reference images required to evaluate the diffusion-coded can additionally be used to generate T2-weighted or T2*-weighted images. This is due to the fact that an optimally short echo time is desirable for the diffusion-weighted acquisitions in order to reduce the signal losses due to the T2 decay, and thus to maximize the signal-to-noise ratio. In single-shot diffusion-coded acquisitions, the image quality is too poor in order to use them as T2- or T2*-weighted images. An additional advantage of an optimally short echo time is that a short repetition time is then possible. Longer echo times, however, are required for T2-weighted acquisitions and T2*-weighted acquisitions since then it is precisely the (long) T2 decay that includes the significant information.